1. The present invention relates to an automatic white balance control circuit and, more particularly, to a picture integration type automatic white balance control circuit.
2. Description of the Prior Art
As shown in FIG. 1, in a conventional automatic white balance control circuit, an optical image is projected by a lens system 51 onto a charge-coupled device (CCD) 52, having complementary color filters, and wherein the image is converted into an electric signal which is supplied to a sample/hold, color separation and automatic gain control (AGC) circuit 53. Color-sequential signals S3, comprising yellow (Ye), green (G) and cyan (Cy) signals, are separated at the sample/hold and color separation portion of circuit 53 by color separation pulses having respective color phases. The separated signals S3(4e), S3(G) and S3(Cy) are supplied through the AGC portion of circuit 53 to an arithmetic circuit 54, in which such signals are converted into three primary color signals, that is, red, green and blue (R,G,B). The color signals R, G and B are supplied to an automatic white balance control circuit which generally comprises variable gain amplifiers 55, 56 and 57, integration circuits 58, 59 and 60, analog-to-digital converters 61, 62 and 63, a controller 68 and digital-to-analog converters 69 and 70. More specifically, the color signals R, G and B are supplied to the variable gain amplifiers 55, 56 and 57, respectively, and the variable gain amplifier 56 has a constant gain level of 1. The color signals R, G and B are also supplied to the integration circuits 58, 59 and 60, respectively, which are adapted to integrate the received color signals and provide respective integrated value signals therefrom. Thus, the output level of the color signal R is integrated at integration circuit 58 and an integrated value signal IR, representing an average level of the color signal R, is supplied to the analog-to-digital converter 61. Integrated value signals IG and IB are similarly obtained from integration circuits 59 and 60, respectively, and are supplied to analog-to-digital converters 62 and 63, respectively. The digitized integrated value signals are supplied to controller 68 which is adapted to calculate the ratio of the integrated value signal IR to the integrated value signal IG and the ratio of the integrated value signal IB to the integrated value IG, that is, the ratios IR/IG and IB/IG, respectively. Reference ratios IRO/IGO and IBO/IGO, obtained while viewing a "white" camera subject under a light source with various color temperatures, are previously calculated and retained by controller 68, and are utilized in generating a blackbody radiation curve CBL, such as is shown in FIG. 2. A tracking range A1 is provided on each side of the blackbody radiation curve CBL.
As illustrated in FIG. 2, the ratios IR0/IGO and IB0/IB0 are inversely proportional to each other, that is, as one of the ratios IR0/IGO and IB0/IGO increases, the other decreases. Further, as the color temperature increases so does the ratio IB0/IG0. On the other hand, as the color temperature decreases the ratio IR0/IG0 increases.
White balancing is only performed when the ratios IR/IG and IB/IG fall within the tracking ranges A1. More specifically, a determination is made, at controller 68, whether the ratios IR/IG and IB/IG obtained from actual picture data are within the tracking ranges A1. If the ratios lie within the tracking range A1, white balancing is effected with the gain levels of the color signals R and B being calculated on the basis of the ratios IR/IG and IB/IG. In other words, digital gain control signals are produced in controller 68 and are supplied as analog control signals DGR and DGB to variable gain amplifiers 55 and 57 through D/A converters 69 and 70, respectively. The gain levels of the amplifiers 55 and 57, which are proportionally controlled by gain control signals DGR and DGB, respectively, can be expressed as follows: EQU gain of the amplifier 55 (Rg)=1/ (IR/IG) EQU gain of the amplifier 57 (Bg)=1/ (IB/IG)
As a result, the output signal levels of the three primary color signals R, G and B from the respective amplifiers 55, 56 and 57 are set equal to each other, that is, (R:G:B= 1:1:1) when the camera is focused on a "white" subject. Further, when the ratios IR/IG and IB/IG lie within the tracking ranges A1, white balancing is performed. On the other hand, when the ratios IR/IG and IB/IG fall outside the tracking ranges A1, white balancing is difficult to achieve, and, in this situation, white balancing is not performed and the gain levels of amplifiers 55 and 57 are maintained at their previous levels.
The three primary color signals R, G and B from variable gain amplifiers 55, 56 and 57 are supplied to a matrix circuit 64 in which, the color signals are converted into color difference signals R-Y and B-Y. The color difference signals R-Y and B-Y and a luminance signal Y suitably applied to a terminal 65 are supplied to an encoder 66 which is adapted to convert the received signals into a color video signal SVD in the NTSC format supplied to an output terminal 67.
When a conventional video camera equipped with the described automatic white balance control circuit is operated with the scene in the field of view illuminated by a light source of varying intensities and color temperatures, such as, a fluorescent lamp, the conventional automatic white balance control circuit may not operate properly. More specifically, the intensity and color temperature of a fluorescent lamp energized from an AC power supply source vary depending upon the instantaneous voltage of such power supply source. The resulting variation in the intensity and color temperature frequently causes the ratios IR/IG and IB/IG to fall outside the tracking ranges A1. As previously mentioned, adjustment of the primary color gain levels by the known automatic white balance circuit is normally effected only when the ratios IR/IG and IB/IG fall within the tracking ranges A1.
Further, when the camera exposure or field frequency and the light source power supply frequency differ, flicker may result, that is, there may be a variation in the brightness of the color image signal from one field to the next. For example, in Japan, if the exposure period of an imager, such as the CCD 52, corresponds to 60 Hz while the standard power supply source frequency is 50 Hz, the amount of light to which the imager is exposed varies between fields as shown in FIG. 3, in which a full wave rectified 50 Hz power supply signal is shown in relation to successive field periods F1, F2, F3 . . . etc. Thus, the produced image signal contains a variation in brightness, or flicker. It is to be appreciated from FIG. 3 that the amount of light to which a scene is exposed in the field F4 is the same as that for the field F1. In other words, the exposure of a field F.sub.n is equal to that of a field F.sub.n+3 which is positioned three fields later so that the flicker has a frequency of 20 Hz.
Since the integration output signals from integration circuits 58, 59 and 60 include the flicker component, the conventional automatic white balance control circuit cannot remove flicker during the balancing operation.